The present invention relates generally to inspection devices, and more particularly to a device which can perform inspections of a substrate by an infrared imaging technique.
Cracks or defects in a substrate can result in substrate failures or failures in the system in which the substrate is placed. When the substrate is used as a solar cell, the crack or defect has the potential to severely limit the power output of the solar panel which contains the defective solar cell. Once a crack has begun, it is highly probable that it will propagate over time to develop into a more significant crack; therefore, it is important to detect not only large but also small cracks. Flight program specifications typically require few or zero cracks in solar panels many square meters in size.
A well-known method for inspecting solar cells involves illuminating the solar cell from the side with a tungsten halogen lamp and imaging the returned light with an infrared camera. Since this technique requires illuminating the solar cell from one side, it typically results in one side of the solar cell being too bright and the other side being too dim such that the solar cell is not uniformly illuminated. This lack of uniform illumination can mask small cracks, thereby leaving them undetected.
This prior art method also typically requires operating the tungsten halogen lamp at a relatively high intensity level in order to provide sufficient illumination of the entire solar cell. Because a tungsten halogen lamp has a quartz envelope, it strongly absorbs energy in the 2 to 5 micron wavelength range of interest. To compensate for such absorption losses, a tungsten halogen lamp often operates at a relatively high temperature resulting in a large fraction of the light output power having shorter wavelengths. The shorter wavelength radiation, especially in the 1.0 to 2.0 micron range can result in glare due to strong reflections of the solar cell cover glass in the 1.0 to 2.0 micron wavelength range. Additionally, this high intensity level adds heat to the solar cell, which can result in thermal expansion of the cell, closing cracks temporarily during the inspection process such that those cracks avoid detection. Excessive heating of the solar cell can also result in ambiguous cell crack detection since the cell re-radiates the heat which is then imaged by the camera. Features in the image can appear as cracks even though no cracks exist.
In addition, the current method cannot objectively and definitively differentiate surface anomalies from substrate cracks. Surface anomalies are mainly caused by the presence of contaminants, such as solvent stains, on the surface of the substrate. The contaminant is generally not harmful to solar cell performance and typically does not result in a rejection of the solar cell. For the current method, the image of a surface anomaly is typically more diffuse that the image of a cell crack; however, in many cases it is difficult to distinguish between a cell crack and a surface anomaly based solely on the appearance of the image. As a result, reinspection of the cell to distinguish between a crack and a surface anomaly is typically required, resulting in increased inspection time and cost. It is desirable to distinguish surface anomalies from cell cracks to avoid any unnecessary rejection of good cells or unnecessary rework of an otherwise good cell.
What is needed therefore is an apparatus and method for detecting defects in a substrate such as a solar cell which provides consistent detection of defects such as cracks, differentiates between types of defects, and, does not excessively heat the substrate.